One aspect of the invention relates to protecting the drain bushing when iron oxide is present in the molten material. In one of its more specific aspects, this invention relates to a method for protecting drain bushings from deterioration due to a cathodic dc bias.
Various platinum and related platinum alloys have been developed which are directed to resisting the heat and chemical attack to which such drain bushings are normally subjected during glass manufacturing. However, when the glass and the batch contain iron oxide, the electrical potential within the furnace can establish an electrochemical reaction with the bushing material. This current, potential within the melt, is normally discharged through the refractory; however, as is common practice, a separate current is carried through the bushing to maintain the molten glass temperature prior to feeding the fiberizing spinner; and, with the presence of iron oxide in the molten material, this current within the drain bushing provides a lower resistance to ground causing the iron oxide to be reduced to iron at the drain bushing. This, in turn, results in grain boundary embrittlement of the platinum/platinum alloy drain bushing and subsequent cracking.
Basalt, which for example, contains in excess of about 8 percent and typically about 15 percent iron oxide, is one material used to form glass and the use of which has resulted in very short bushing life due to such cracking.
During the production of glass fibers, the drain bushing operates at temperatures in excess of 2000.degree. F. (1093.degree. C.). Since melted glass is introduced into the drain bushing and must remain in the melted state during its residence, an ac vo1tage is app1ied to the drain bushing to heat it and assist in contro11ing the f1ow rate of the glass melt through the bushing. Inasmuch as such bushings are fabricated from platinum or platinum alloys, which are expensive and the bushings themselves are expensive to cast, there is a need in the art to avoid the destruction of these bushings. The mode of cracking of a platinum or platinum alloy drain bushing is the same as that which platinum experiences when it is cathodically polarized in molten glass; that is, the drain bushing becomes part of an ac circuit when current is applied for heating the bushing, but it does not remain at the same potential as the g1ass and the refractory, so a dc current is established on the bushing. When a cathodic dc bias is developed on the bushing in this manner, in a basalt or other high iron oxide type glass, the iron oxide within the glass melt is reduced to iron, causing grain boundary embrittlement of the platinum alloy and the bushing life is substantially shorter than economical processing can tolerate. This invention is therefore directed to the solution of that problem.
This cracking problem, as indicated above, is characteristic of high iron oxide bearing glass batch materials, e.g., those containing greater than about 8 percent by weight of iron oxide. It will be noted that no distinction is made herein between glass and mineral fibers, that is, glass fibers comprehend mineral fibers unless expressly indicated to the contrary. Glass batch containing iron oxide in such amounts to degrade the bushing is referred to herein as containing iron oxide in a bushing degrading amount.
The present invention has, as an objective, retarding bushing degradation by glass batch containing iron oxide in a bushing degrading amount. Essentially, the problem is solved by substantially eliminating the passage of current directly through the bushing for heating. In lieu of directly heating, there is employed a conductive heating means.